Police found a bundle of dynamite in one of Paris's best-known department stores yesterday after a warning letter from a group threatening to attack stores if French soldiers are not withdrawn from Afghanistan.

Five sticks of dynamite tied together totalling 500g but with no detonator were hidden in the cistern of a third-floor toilet of the men's department at Printemps, the luxury department store on Boulevard Hausmann, one of the city's main shopping streets. Thousands of shoppers were evacuated at the height of the Christmas rush, and roads were closed as police and sniffer dogs searched the building.

At the scene, France's interior minister, Michèle Alliot-Marie, said the dynamite was "relatively old", there was no detonator and "no risk of explosion".

Police acted after the French news agency Agence France-Presse received a letter in the post yesterday morning from the Afghan Revolutionary Front. The letter, written in inaccurate French and postmarked from north-east Paris on Monday, warned that several bombs had been placed in the Printemps store.

"Tell your president to withdraw the troops from our country before February 2009 or we will undertake more action in your capitalist department stores this time without warning," it said. The interior minister said the group was "totally unknown" to French security services and investigators would be "cautious" over claims in the letter.

AFP reported that last Wednesday an anonymous caller rang the agency from a phone-box in Paris saying a bomb was about to go off at Printemps. The store was evacuated but it was a false alarm.

President Nicolas Sarkozy supports western involvement in Afghanistan and this year increased the number of French troops serving in the Nato contingent to more than 2,600. He said yesterday: "We need to be vigilant about terrorism ... We cannot compromise with terrorists."

A police official said several aspects of the incident did not bear the hallmarks of Islamist terrorism: the warning, the type of explosives used, the language in the letter and the name used by the group.

During the evacuation there was a mood of dread and confusion outside the store, which attracts 100,000 people a day. "I just left my vacuum cleaner and ran. I didn't have time to get my handbag, I was just thinking of survival," said a cleaner.

Marie Mantoux, a Parisian midwife, was resolved to finish her shopping. "I've got a Printemps advantage card so I don't want to go anywhere else. And you can't give in to fear, or the city would shut down. France hasn't done enough to help the civilians of Afghanistan, maybe that's what this is about."

By 2pm shoppers were allowed back into the store.

France's involvement in Afghanistan sparked public disapproval in August when 10 soldiers were killed in a Taliban ambush east of Kabul, the biggest single death toll for the army in 25 years.